An investigation into control techniques for cascaded plants with buffering, to minimise the influence of process disturbances and to maximise the process yield

ENGLISH ABSTRACT: The Coal to Liquid facility, Sasol, Secunda operates as a train of processes. Disturbances
and capacity restrictions can occur throughout the plant and the throughput
fluctuates whenever disturbances occur. When capacity restrictions occur in a subplant
and more substances enter the sub-plant than can be processed, the extra substances
are flared or dumped and therefore lost. To reduce losses and extra costs and
to maximise the throughput of the whole plant, supervisory control is implemented over
the whole plant system.
Each process in the process train is controlled with regulatory controllers and the overall
process is then controlled with a supervisory controller. These two sets of controllers
operate in two different layers of control, with the regulatory controllers the faster inner
layer. The supervisory control is the outer layer of the two control layers. The
supervisory controller takes over the work of the human operator by deciding on the
changes in total throughput as well as the set points for each individual process. These
set points for each process are then followed with the regulatory controllers. For the
regulatory control of the system, different control methods are investigated and compared.
The different control methods that are looked at are PI control, Linearised State
Feedback control, Fuzzy Logic control and Model Reference Adaptive Control.
After an investigation into the various control methods Fuzzy Logic control was chosen
for the regulatory as well as the supervisory control levels. Fuzzy Logic control is a rule
based control method. Fuzzy variables are everyday terms such as very slow or nearly
full. These terms are easy to understand by the operator and multi-variable control is
possible with Fuzzy Logic control without an accurate mathematical representation of
the system. These facts made Fuzzy Logic control ideal for this implementation.
To improve the profit of the Coal to Liquid facility the throughput was maximised. The
combination of regulatory and supervisory controllers minimised losses and rejected
disturbances. This resulted in a smoother output with maximum profit.